1. Field of the Invention
The present invention relates generally to measuring a time delay of an event relative to a reference time for providing the range to a target. More particularly, the invention relates to a method and apparatus for measuring time delays between a reflected pulse from each of multiple targets relative to a reference pulse.
2. State of the Art
Devices for measuring a time delay using, for example, opto-electronic, electronic and ultrasonic range finders are known. Opto-electronic range finders, such as laser range finders, measure a time-of-flight of a transmitted laser pulse; that is, the time between transmission of the pulse and detection of a reflected laser return pulse from a target. To determine a time-of-flight of the transmitted laser pulse, a counter is typically started upon the emission of the laser pulse, and is then stopped upon receipt of the reflected pulse.
In addition, where multiple targets in the field of view are to be detected, devices have been developed to measure the time-of-flight to the additional targets. For example, a known opto-electronic range finder includes a counter which is started upon emission of a light pulse, and then stopped upon receipt of a reflected pulse. Once the counter has been stopped, the count information is transferred from the counter to an intermediate storage location. The counter then continues to count until the next reflected pulse is received.
Despite an ability to detect multiple reflected pulses, such devices suffer significant drawbacks. For example, the range finder device must be capable of transferring data from the counter to the intermediate storage in a time interval which is less than the resolution of the counter. That is, data must be transferred to the intermediate storage within one clock cycle so that the counter output does not change state during the transfer of data. Any practical implementation of such a range finder therefore requires use of a synchronous counter. However, it is difficult to implement a synchronous counter with a data width of 8 bits or more, and with counter resolutions on the order of 500 picoseconds.
Another known opto-electronic range finding device includes multiple counters. Each of the counters is started upon the emission of a light pulse, and each counter is stopped in sequence as multiple reflected pulses are received. However, this device suffers the drawback of requiring multiple counters, each having a wide data width and high resolution. The large area and very high power dissipation required for the use of multiple counters limits the number of targets which can be detected in a practical embodiment.
U.S. Pat. No. 5,353,228 (Geiss et al) discloses another known apparatus for detecting a range to multiple targets in a field of view. This patent describes dividing a predetermined measurement cycle into multiple time intervals. A sequence of distances is associated with the round-trip transit time receivable within each time interval, and a digital memory is used to store the presence or absence of a target at each interval. A disadvantage of this device is that its maximum resolution is a function of the time required to write information to the digital memory. In addition, this device is limited to a measurement cycle which must detect the presence or absence of a reflected target pulse within every designated time interval. Consequently, when the number of time intervals in the measurement cycle is increased to improve range or resolution, the number of locations in the storage device must be increased linearly. For example, increasing the measurement cycle by a factor of two, requires an attendant increase in the size of the digital memory by a factor of two. Accordingly, this device is impractical when high resolution (that is, a short time interval) and/or a large measurement cycle are required. Further, because this device stores an entire history of reflected target pulses for a given measurement cycle before any information is read out to a signal evaluation device, it is unsuited for real-time operation.
Other conventional range finding devices are premised on the use of range gating, wherein a counter is enabled for only a small interval of time. For example, to locate a first target which is expected to be within ten meters from the transmitter, the counter is enabled to detect a reflected pulse within a period of time which corresponds to a distance of ten meters. If no target is detected within this range, then the counter is enabled to detect reflected pulses from a target within a range of 10-20 meters. This process continues for each gated range, until all designated ranges have been examined. A disadvantage of range gating is that an increase in resolution can significantly increase the time required to perform a single measurement cycle. For example, if each specified interval possesses one meter resolution, then a measurement sequence for a range of 2000 meters would require 2000 separate measurements. Such operation can be extremely timely, inefficient and unsuitable for real-time data acquisition.
U.S. Pat. No. 4,477,184 (Endo) discloses a range finding device which suffers drawbacks similar to those described with respect to range gating. Here, a scanning laser is used to detect targets across an entire field of view. To achieve high resolution, each segmented portion of the field of view is relatively small. Because the time required to scan the entire field of view increases in proportion to the resolution desired, the disclosed device is unsuitable for achieving high resolution in real-time.
Accordingly, it would be desirable to provide a method and apparatus for determining ranges to multiple targets, in real-time, using a system having high resolution over a large maximum range. In so doing, it would be desirable to provide a practical, cost-effective system which can be easily reconfigured by the user.